1. Field of the Invention
The present invention relates to a piezoelectric MEMS (Micro-electro-mechanical System) element and a tunable filter, particularly, to a variable capacity having a piezoelectric driving mechanism utilizing a piezoelectric thin film.
2. Description of the Related Art
A variable capacity/diode utilizing a change in the thickness of the depletion layer in the PN junction is known as a conventional variable capacity or capacitor. However, the variable capacity/diode has a capacitance varying range that is only about 5 times as much as the reference capacitance value, and the quality coefficient (Q value) of the variable capacity/diode denoting the smallness of the loss is small, i.e., about 20 to 30, and, thus, the use of the variable capacity/diode is currently limited.
On the other hand, a variable capacity or capacitor prepared by the MEMS technology attracts attention in recent years. For example, a variable capacity is exemplified in Japanese Patent Disclosure (Kokai) No. 2003-258502. In the variable capacity disclosed in this prior art, a movable body is arranged upward of a coplanar line formed on a substrate, and a movable electrode is mounted to the movable body so as to face the coplanar line. A DC current is applied between the movable electrode and a stationary electrode so as to displace the movable body toward the stationary electrode and, thus, a capacitance between the movable electrode and the coplanar line is varied.
Recently, a MEMS type variable capacity utilizing the piezoelectric reaction as the driving force of a movable beam attracts attention. This variable capacity permits wide and continuous variation of the distance between the movable electrode and the stationary electrode, leading to various merits. For example, it is possible to obtain a high rate of change of capacitance. Also, since air or a gas is used as the dielectric material, it is possible to obtain a large Q value.
Also, it is possible for the variable capacity to be of a capacity type structure in which the movable electrode is brought into contact with the stationary electrode with a very thin dielectric film interposed therebetween or to be of a DC type switch structure in which the movable electrode is brought into direct contact with the stationary electrode. The switch manufactured by the MEMS technology described above permits a low on-resistance and high insulating separating properties in the off-time, compared with a semiconductor switch and, thus, attracts attentions very much.
However, the MEMS type variable capacity has a long and thin beam structure supported in the air and includes a piezoelectric layer sandwiched between upper and lower electrodes. The particular construction gives rise to the problems that a slight residual stress in the material constituting the beam causes the variable capacity to be warped in the vertical direction and that a slight difference in the thickness and the properties of the material constituting the beam causes an amount of displacement generated by the voltage application to be changed. It follows that it is very difficult to maintain the capacitance value before and after the voltage application as designed in the variable capacity and to maintain the same change in the capacitance of a large number of variable capacities.